Add support for serializing/deserializing of SkDrawable

src/core/SkValidatingReadBuffer.cpp

Index: src/core/SkValidatingReadBuffer.cpp
diff --git a/src/core/SkValidatingReadBuffer.cpp b/src/core/SkValidatingReadBuffer.cpp
index bc5611a3e382902878ca99256f6d26ff0e80c3c2..a45c6ca9b0623f483e802667401c307f3387b8f9 100644
--- a/src/core/SkValidatingReadBuffer.cpp
+++ b/src/core/SkValidatingReadBuffer.cpp
@@ -246,6 +246,12 @@ SkFlattenable* SkValidatingReadBuffer::readFlattenable(SkFlattenable::Type type)
     SkFlattenable* obj = nullptr;
     uint32_t sizeRecorded = this->readUInt();
     if (factory) {
+        // Check if a custom Factory has been specified for this flattenable.
+        SkFlattenable::Factory* customFactoryPtr = getCustomFactory(factory);
+        if (customFactoryPtr) {
+            factory = *customFactoryPtr;
+        }
+
         size_t offset = fReader.offset();
         obj = (*factory)(*this);
         // check that we read the amount we expected